High-concentration hydrogels of κ-carrageenan prepared using subcritical water

κ-Carrageenan hydrogels have been prepared at very high concentrations beyond the previous limit of conventional κ-carrageenan hydrogels. By dissolving κ-carrageenan using subcritical water at 150 °C, homogeneous translucent hydrogels have been obtained from 15 wt% to 40 wt%. The high-concentration hydrogels have exceptionally high Young's modulus (E₀) ranging 10⁶-10⁷ Pa and exhibit an unknown concentration (c)-dependence of E₀ ∝ c^1.0-1.1. The distinctive mechanical properties of the high-concentration hydrogels are also demonstrated by modifying the stress (σ)-strain (ε) curves. Curves of σ / E₀ versus ε for the high-concentration hydrogels seem to fall on a single curve that is remarkably different from that for the conventional 3 wt% hydrogel. The gel-to-sol transition temperature (T_m) proves high thermal resistance of the high-concentration hydrogels; T_m increases with c and exceeds 100 °C at 30 wt% and 40 wt%. It has been confirmed that separated pieces of high-concentration hydrogels are unified into a single disk above T_m. The endothermic enthalpy of the gel-to-sol transition per unit weight of κ-carrageenan is almost constant regardless of c among the high-concentration hydrogels, which is consistent with the claim that the high-concentration hydrogels have similar distinctive network structure.